1. Field of the Invention
The present invention relates to a method of forming a diamond or diamond containing carbon film, under a lower pressure region. More specifically the present invention is directed to a particular selection of reactive gases for forming the carbon layer by CVD method, and further relates to an improvement of an introducing method of reactive gases into CVD apparatuses.
2. Description of the Prior Art
Up to the present time, as a method of forming a diamond film or a diamond containing carbon film, a thermal CVD (Chemical Vapor Deposition) method has been most frequently utilized. This is a method, as shown for example in FIG. 1, to form a diamond film or a diamond containing carbon film on a substrate, by causing a reactive gas introduced into a quartz reaction vessel 1 through a gas inlet 2 and outlet 5 to chemically react on the substrate 4, which is heated at 400.degree.-1300.degree. C. by releasing thermoelectrons from a metallic tungsten or tantalum filament S heated by electric current at 1500.degree.-3000.degree. C. At this time, the pressure in the reaction vessel is kept at 1-350 Torr. Therefore, it is cheap and easy to handle the thermal CVD method.
As another method of forming a diamond film or a diamond containing carbon film, it has been known to use a microwave plasma CVD method. This is, as shown for example in FIG. 2, to introduce a reactive gas through an upper part of a quartz reaction tube 1 which is inserted into one part of a microwave guide 6, and to effect a vacuum exhausting through a lower portion of the quartz reaction tube. In this method, the microwave having 2.45 GHz frequency is regularly employed, and the pressure in the reaction tube is kept at 10-200 Torr.
Further, it has been known for forming a diamond film or a diamond containing carbon film to use a mutual action between microwave and magnetic field. There are two ways in this type of the CVD methods, one is called a magnetic field-microwave plasma CVD method utilizing MCR (Mixed Cyclotron Resonance) which occurs when the pressure is 0.1 Torr or higher as disclosed in U.S. Pat. No. 5,015,494, having Ser. No. 329,877 issued to Yamazaki, and the other is the ECR plasma method using ECR (Electron Cyclotron Resonance), the phenomenon arising in case of such very low reaction pressure as less than 0.1 Torr.
FIG. 3 shows a schematic diagram of an apparatus used for the magnetic field-microwave plasma CVD applying MCR, in which a reactive gas introduced into a reaction chamber through a gas inlet 2 will be effectively excited by using a mutual action between a magnetic field induced by a electromagnetic coil 7 and a microwave introduced into the reaction chamber from a microwave guide 8, resulting in that a diamond film or a diamond containing carbon film will be formed on a substrate 4. The substrate is under the control of temperature, by heating the substrate holder, and also it is possible to add a floating field 8 to the substrate. Such a CVD method as explained above, has a drawback that it is inevitably inefficient to form a large area film, and the film-formation is at most limited to .phi.100 mm substrate, being caused by the mass and the operational nature of the reaction chamber.
On the other hand, the shape of an ECR plasma CVD apparatus is almost the same as the above stated magnetic field-microwave plasma CVD apparatus. But according to the ECR plasma apparatus, the plasma can spread more as compared with the magnetic field-microwave CVD apparatus, arising from such a very low reaction pressure as less than 0.1 Torr, thus making it possible to form a film on a larger substrate. For this reason, almost all of the ECR plasma CVD apparatuses are a type of the "Deposition-Down" as illustrated in FIG. 4. In general, a reactive gas is inputted to the apparatus through a gas inlet 2. Alternatively, a dilution gas is inputted through the gas inlet 2, and to input a raw material gas through a gas inlet 9 or 10. By the ECR plasma CVD method, only the carbon film containing less proportion of diamond can be formed. Still the diamond film can not be formed.
Further, in case where a mixed gas of a raw material gas and a dilution gas is introduced through the gas inlet 2, it is impossible to provide a sufficient amount of active particles containing carbon around the substrate, since the active particles happen to recombine before they reach to the substrate, the result being that the deposition rate will be inferior to that of the magnetic field-microwave plasma CVD method. Also in case where a raw material gas is inputted through the gas inlet 9 and a dilution gas is inputted through the gas inlet 2, it is impossible to supply the substrate with sufficient raw material gas, as a boundary will be made at the verge of the spread plasma.
Still further, in case where a raw material gas through gas inlet 10 and a dilution gas through gas inlet 2 are inputted respectively, while it can be attained that the deposition rate is maintained at 0.3 .mu.m/hour which is regarded as the maximum value by ECR plasma CVD method, the uniformity of the formed film is not very good, for example, there exists a portion in the film at which the thickness thereof is only 30% of the maximum thickness thereof. We understand this is because the plasma is interrupted by the gas introducing conduit.
As a gas utilized in the formation, a raw material which is in a gas phase or a liquid phase at normal temperature and pressure (i.e. room temperature and atmospheric pressure) is utilized. The raw material which is gaseous at normal temperature and pressure has an advantage that it is easy to control its flowing amount. For instance, methane, ethylene, carbon monoxide, acetylene, hydrogen, carbon dioxide, and argon are used. On the other hand, as to the raw material which is in liquid state at normal temperature and pressure, there are two kinds of gases which are usable or not, depending upon their vapor pressure curve and upon a reaction pressure during the formation. For example, methanol, ethanol, acetone, and water are utilized as a liquid raw material for forming diamond.
In general, the deposition rate is not higher than 1 .mu.m/hour in the case of the magnetic field-microwave plasma CVD method using the MCR, and not higher than 0.3 .mu.m/hour in the case of the ECR CVD method.